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When it Comes to Seeing Atoms, Blurrier is Better

One way to see atoms is to focus a beam of high-energy electrons to a tiny spot smaller than an atom and scan it around. The clarity of the resulting image is a tradeoff between collecting sufficient electrons to get a good signal, but too many electrons and the sample will be damaged.

EMPAD Sample Schematic

Figure: Schematic of ptychography with defocused probe electrons. a) The experimental setup shows that the focus of the electron probe is downstream from the sample at the distance defined by the defocus value. The diffraction pattern on the detector (EMPAD) shows a shadow image in the bright-field disk. b) A diagram of the scan procedure. c) Three simulated diffraction patterns when the probe is illuminated at the positions circled in (b), which show the shadow images in the bright-field disk shifting accordingly with the probe position.

 

Here, researchers of PARADIM’s in-house team demonstrate that deliberately defocusing the beam can lead to faster, higher-resolution and more dose-efficient imaging when a more sophisticated detector is used. Instead of discarding or averaging the scattered electrons, the Cornell EMPAD detector collects every transmitted electron as a function of angle for every beam position, giving extra information that is used to reconstruct the original object.

Compared with conventional atomic-resolution electron microscopy techniques, the new approach simultaneously provides a four-times-faster acquisition, with double the information limit at the same dose, or up to a fifty-fold reduction in dose at the same resolution.

What Has Been Achieved: Determining the local atomic arrangement of complex nanostructures can provide fundamental insights into the properties of materials. Compared to traditional metals and semiconductors, newer materials systems as metal-organic frameworks and organic perovskites are more radiation sensitive, requiring more dose-efficient imaging techniques in order to allow high-resolution imaging with comparable level of detail. Solving the structure of biological macromolecules or small molecular at atomic level is even more challenging. The main problem is that, as a consequence of Poisson statistics, the required illumination dose is inversely proportional to the square of the spatial resolution, and thus improving spatial resolution means quadratically higher doses. The increased dose may destroy the structure of the sample before sufficient image signal-to-noise is reached. The widely adopted atomic-resolution imaging methods in scanning transmission electron microscopy (STEM), such as annular dark-field (ADF) or coherent bright-field (cBF) imaging, are intrinsically dose inefficient as they use only a small fraction of the scattered electrons, being constructed via a simple integration of a limited portion of phase space. Therefore, conventional STEM imaging methods usually cannot achieve sub-angstrom resolution or even atomic-resolution for electron radiation-sensitive materials. Meanwhile, high-precision measurement of local atomic positions is also fundamentally hindered by the poor signal-to-noise ratio of ADF images from electron-radiation sensitive or weakly scattering samples, such as monolayer 2D materials. Picometer precision via ADF imaging can only be achieved in electron-radiation-robust and strongly scattering bulk samples. Electron ptychography, however, can potentially use the entire diffraction patterns either via a Wigner-distribution deconvolution (WDD) or iterative algorithms in a way that can account for the probe damping effect and extract the electrostatic potential of the sample. In particular, ptychography has now surpassed the resolution of the best physical lenses, reaching deep sub-angstrom resolution.

Importance of the Achievement: Demonstration of extremely low-dose imaging by electron ptychography, enabling the study of electron radiation-sensitive materials including biological macromolecules.

Unique Feature(s) of the MIP that Enabled this Achievement: PARADIM is dedicated to advancing characterization capabilities for electron microscopy and making them available to users.  This highlight made use of PARADIM’s unique EMPAD detector and has developed image analysis routines that are open and available to all PARADIM users.

Scientists of PARADIM’s In-House Research team performed the work with contributions from the Paul Scherrer Institute (Switzerland), the King Abdullah University of Science and Technology (Saudi Arabia), and the Chang-Gung University (Taiwan).

Full References: Z. Chen, M. Odstrcil, Y. Jiang, Y. Han, M.-H. Chiu, L.-J. Li, and D.A. Muller, “Mixed-state electron ptychography enables sub-angstrom resolution imaging with picometer precision at low dose,” Nat. Commun. 11, 2994 (2020).

The work is featured in the Cornell Chronicle.

Data Availability: Raw data are available at https://doi.org/10.34863/G4WA-0J57 provided through the PARADIM Data Collective.